If you know x86 assembly, you will notice that it relies on a GenuineIntel CPU and for CPUID leaf 0 to return a value less than 3.
As for the .NET Framework 1.1 problems, the way to determine if SSE is supported is to first use CPUID to determine if the SSE bit is set. But there is also an extra step. Without CR4.OSFXSR set, SSE instructions will cause #UD. This can be caught on Windows as a SEH exception. My guess is that .NET 1.1 is not doing that, which is why it crashes without INTLFXSR.SYS properly loaded.